Pneumatic tires of course require sufficient steering stability on a dry road surface. Depending on the season and changes in the weather, however, such tires may also be driven on a snowy road surface and thus require on-snow performance as well.
One effective method for improving steering stability on a dry road surface is, for example, to increase the tire ground contact area of the tread surface that contacts the road surface, thereby increasing the rigidity of the land portions in the entire tread.
On the other hand, one effective method for improving on-snow performance is to set a high negative ratio by, for example, increasing the number of tread grooves formed on the tread surface or increasing the groove width of the tread grooves, thereby making it easier for snow to enter the tread grooves. Snow in the grooves is then packed during tire ground contact so as to be compressed into a snow column, resulting in the tire gripping the snowy road surface firmly without slipping.
For this reason, there is generally a tradeoff between steering stability on a dry road surface and on-snow performance.
Patent Literature 1, previously submitted by the present applicant, is an example of a conventional pneumatic tire developed to achieve a balance between steering stability on a dry road surface and on-snow performance.
The pneumatic tire disclosed in Patent Literature 1 is a pneumatic tire having, on the tread surface portion, a plurality of circumferential main grooves extending in the tire circumferential direction and a plurality of land portion rows defined by the circumferential main grooves. In at least one of the plurality of land portion rows, a circumferential auxiliary groove is formed extending in the tire circumferential direction at a location other than the tire equator. The circumferential auxiliary groove has a narrower width and a shallower depth than those of the circumferential main grooves. Of the two side faces defined by this circumferential auxiliary groove, the side face on the tire equator side extends along the normal direction of the tread surface portion or is inclined to the tire equator side by 5° or less with respect to the normal direction. The other side face on the side opposite the tire equator side is inclined to the side opposite the tire equator side by 10° to 30° with respect to the normal direction. This structure improves turning performance on snowy roads without decreasing steering stability or quietness when driving on a dry road surface.
The applicant subsequently examined how to achieve both steering stability on a dry road surface and on-snow performance to an even greater degree and discovered the following. In the tire of Patent Literature 1, the circumferential auxiliary groove needs to be provided in at least one of the central land portion and a outer land portion. Therefore, the circumferential auxiliary groove may be provided in the outer land portion. In this case, although the relationship with the tire ground contact width is a factor, there is a high probability that the circumferential auxiliary groove provided in one of the outer land portions will be outside the tire ground contact region when turning on a snowy road surface. In such a case, if the other side face of the circumferential auxiliary groove on the side opposite the tire equator side is inclined to the side opposite the tire equator side by 10° to 30° with respect to the normal direction, snow cannot be sufficiently drawn into the circumferential auxiliary groove and formed into a snow column, leading to the risk of slipping and also reducing the steering stability due to a reduction in the tire ground contact area when driving straight on a dry road surface. In particular when the tires are mounted on the front and rear wheels and the tire applied load differs between the front and rear tires, for example when the tire applied load is higher on the front tires than on the rear tires, the circumferential auxiliary grooves on the front tires are within the tire ground contact region whereas the circumferential auxiliary grooves on the rear tires are outside of the tire ground contact region. As a result, a difference in grip on a snowy road surface occurs between the front and rear tires, which worsens the front and rear balance and makes the above tendency to slip when turning on a snowy road surface pronounced.